Recently, the development of so-called low-emission vehicles such as fuel cell vehicles, electric vehicles and hybrid vehicles has been progressed. Particularly, hybrid vehicles are becoming popular both at home and abroad. In the hybrid vehicle and the like, when the voltage is stepped down from the battery voltage to the voltage for electrical equipment, or when a motor or the like is inverter-controlled, conversion from direct current to high frequency alternating current is performed through a switching power supply and the like.
A circuit of the switching power supply as described above is provided with a reactor constituted by a core (magnetic core) and a coil wound around the core. As to the performance of the reactor, the reactor is required to be of small size and have a low loss and low noise and, in addition, it is required to have stable inductance characteristics in a wide direct current range, that is, to have excellent direct current superposition characteristics. Thus, as the core for the reactor, it is preferable to use a core having a low iron loss and a stable magnetic permeability from a low magnetic field to a high magnetic field, that is, a core having excellent constancy in magnetic permeability characteristics.
In general, a core for a reactor is formed of a material such as silicon steel sheet, an amorphous thin band, oxide ferrite and the like, and the cores formed of these materials are produced by stacking plate materials, powder compacting, power compact sintering, or the like. In order to improve the direct current superposition characteristics, there is also an occasion to provide a suitable space (gap) in a magnetic path of the core to adjust an apparent magnetic permeability.
With increasing output of the motor, a core for a reactor or the like has been required to be used on a high current/high magnetic field side. In such a core for a reactor, it is preferable that the differential magnetic permeability is not reduced even on the high magnetic field side, that is, the core has an excellent constancy in magnetic permeability. However, since the core formed of a material such as silicon steel sheet, an amorphous thin band and oxide ferrite is a material having a high magnetic permeability, the magnetic flux density is saturated on the high magnetic field side, and the differential magnetic permeability, that is an inclination of a tangent of a magnetization curve, is reduced. If such a core with less constant magnetic permeability is to be used in a reactor, it is necessary to design the core in such a manner that a thickness of the gap provided in the core is increased or the number of the gap portions is increased. However, such a design of the core causes generation of a leakage magnetic flux, an increase in loss, an increase in noise and an increase in size of the reactor, and the resultant core is not preferable to mount on a vehicle in which a fuel efficiency is required or the mounting space is limited.
As a core whose material structure has unique characteristics, there is a powder magnetic core produced by compacting a powder of soft magnetic metal such as iron. In the powder magnetic core, a material yield at the time of production is high, as compared with a laminated magnetic core formed of silicon steel sheet or the like, and the material cost can be reduced. Further, the powder magnetic core has a high degree of freedom of the shape, and the characteristics can be thus improved by optimally designing the magnetic core shape. Furthermore, electrical insulation between the metal powder particles is possibly improved by mixing an electrical insulating material such as organic resins and an inorganic powders into the metal powder, or by providing an electrical insulating coating on the surface of the metal powder, whereby eddy-current loss of the magnetic core can be significantly reduced and excellent magnetic properties can be obtained especially in a high-frequency region. Based on these characteristics, the powder magnetic core has attracted attention as the core for a reactor.
As a method of producing the powder magnetic core, there is a method of compacting a mixed powder prepared by adding a thermosetting resin powder to a soft magnetic powder having an inorganic insulating coating formed on its surface and subjecting the powder compact to a resin curing treatment (for example, see Patent Citation 1). Recently, the iron loss of the powder magnetic core is required to be further reduced, and heat treatment is applied to the powder magnetic core to mitigate distortion due to compression forming of powder, so that hysteresis loss is reduced (for example, see Patent Citation 2).